Does A Low Fat Diet Prevent Cancer?

Review of JAMA. 2006;295(6):629-42.

This article addresses a thoughtful question that was submitted by one of our website users. The website user asked, "I just read about a big research study that did not show any health benefits from eating a low-fat diet. I thought a low-fat diet could reduce cancer risk. Why didn't this study show any health benefits for women who are eating a low-fat diet?" We discuss the results of this recent, important research study on the potential health benefits of a low-fat diet. We describe how this new information can be incorporated into your own cancer-fighting nutrition plan.

Note: Numbers appearing at the end of sentences indicate research references. References are listed at the end of each article.

Website User Question

"I just read about a big research study that did not show any health benefits from eating a low-fat diet. I thought a low-fat diet could reduce cancer risk. Why didn't this study show any health benefits for women who are eating a low-fat diet?"

Caring4Cancer Response

The study that you have heard so much about comes from a research project called the Women's Health Initiative (WHI) (1,2). This project was set up to follow 48,835 women for 9 years. The women were 50 to 79 years old at the beginning of the WHI project. There are several different studies that are a part of the overall WHI project. The goal of the nutrition study that you've read about was to find out if a low-fat diet with plenty of vegetables and fruit could lower the risk of breast cancer, colon cancer, and heart disease in women (3). The first results from this low-fat diet study were released just last week. These results were big news for several reasons:

• A lot of time and money has been invested into this study.
• This is one of the first controlled diet trials looking at the possible health benefits of a low-fat diet. A controlled diet trial is when some of the people in the study are randomly assigned to follow a specific diet. The health of these people then is compared to other people who are not following the diet. This allows researchers to get a better idea of the health effects of certain diets. It is a better way to study the connection between nutrition and health than simply observing what people eat on their own and asking them about it.
• At first glance, the results of this study seem to be the exact opposite of what people would expect. This study seems to tell us that eating a low-fat diet that contains plenty of vegetables and fruit does not reduce the risk of breast cancer, colon cancer, or heart disease.

What Did The Researchers Study?

Sometimes, what researchers want to study and what they actually do study are two very different things.

The researchers wanted the women in the low-fat diet to do the following: 

• Eat no more than 20% of their total calories as fat.
• Eat a minimum of 5 servings of vegetables and fruit every day.
• Eat a minimum of 6 servings of grains every day.
• Eat no more than 7% of their total calories as saturated fat.

What Did The Women Really Eat?

The women in the low-fat diet group actually ate the following:

Total Fat in the Diet

• By the end of the first year of the study, about 31% of the women in the low-fat group were eating 20% of their calories from fat. This means that two out of three (69%) of the women did NOT meet the low-fat diet goals.
• By the end of the sixth year of the study, about 14% of the women in the low-fat diet group were eating 20% of their calories from fat. This means that 86% of the women did NOT meet the low-fat diet goals of the study after 6 years.
• By the end of the first year of the study, the women in the low-fat diet group were eating an average of 24% of total calories as fat.
• By the end of the sixth year of the study, the women in the low-fat diet group were eating an average of 29% of total calories as fat.

In summary, only a very small number of the women assigned to eat the low-fat diet actually followed the diet guidelines. By the sixth year of the study, only 14% of the women in the low-fat diet group were eating the prescribed low-fat diet! This means 86% of the women did not follow the diet! On average, the women in the low-fat diet group were eating 29% of their calories from fat! This is NOT a low-fat diet. This barely meets the government recommendation for the general public to eat no more than 30% of total calories as fat.

Vegetables & Fruit

• By the end of both the first year and the sixth year of the study, the women in the low-fat diet group were eating an average of 5 servings of vegetables and fruit per day.
• By the end of the first year of the study, the women in the low-fat diet group were eating an average of 1.2 more servings per day of vegetables & fruit than the women in the normal diet group.
• By the end of the sixth year of the study, the women in the low-fat diet group were eating an average of 1.1 more servings per day of vegetables & fruit than the women in the normal diet group.

In summary, the women in the low-fat diet group ate about 5 servings of vegetables and fruit per day, as instructed. However, they only were eating about 1 more serving of vegetables and fruit per day than the women in the regular diet group. In other words, everyone in the study ate almost the same amount of vegetables and fruit.

Grains

• By the end of the first year of the study, the women in the low-fat diet group were eating an average of 5 servings of grains per day. By the end of the sixth year, they were eating an average of 4 servings of grains per day.
• By the end of the first year of the study, the women in the low-fat diet group were eating an average of about 0.9 more servings per day of grains than the women in the normal diet group.
• By the end of the sixth year of the study, the women in the low-fat diet group were eating an average of about 0.4 more servings per day of grains than the women in the normal diet group.

In summary, the women in the low-fat diet group did not meet the goal of eating 6 servings of grains per day. As well, they did not eat substantially more grains than the women in the regular diet group.

Nutrient Levels in the Body

The researchers took blood samples from a small group of women in the study. They wanted to measure the levels of carotenes in each woman's blood. Carotenes are a group of nutrients that are found in vegetables and fruit. Levels of carotenes in a person's blood are a good way to tell how many vegetables and fruit that person regularly eats (4-16). The more vegetables and fruit a person regularly eats, the higher her blood levels of carotenes will be.

After the first and the third years of the study, there were virtually NO differences in the blood levels of carotenes between the women in the low-fat diet group and the women in the regular diet group. In other words, based on blood levels of nutrients, the women in the low-fat group probably were eating about the same amount of vegetables and fruit as the women in the regular diet group.

Saturated Fat

After one year, the women in the low-fat diet group were eating an average of 8.1% of calories from saturated fat. This means that the majority of the women in the low-fat diet group did not meet the goal of eating less than 7% of calories as saturated fat.

New Information About Nutrition & Disease

The WHI project was designed over a decade ago. Since that time, research has found that type of fat in the diet may be more important than amount of fat in the diet. Health experts now believe that eating the right kinds of fat may play a bigger role in our health.

The Time Frame

This study followed the women for an average of 8.1 years. In terms of cancer and heart disease, this is not a long time. The damage in the body that can lead to cancer and heart disease can take several decades to occur. Even if the women in the low-fat diet group followed the diet exactly, 8 years may not be enough time to see a benefit.

Putting It All Together

The bottom line on this study is that women who do not follow a low-fat diet obviously will not get any health benefits from that low-fat diet! Adding 1 serving of vegetables and fruit per day to the diet may not make much of a difference in overall health either. As well, the WHI diet study does not provide enough information to answer the question "Does a low-fat diet lower the risk of cancer and heart disease?"

The women in the low-fat diet group ate an average of 29% of calories from fat. This is not a low-fat diet. They ate 1 more serving of vegetables and fruit per day compared to the regular diet group. They ate less than 1 more serving of grains per day compared to the regular diet group. These differences are too small to allow for meaningful comparisons. There just isn't enough difference in the way these two groups of women were eating to see much of an effect on health!

After learning more about this study, we may want to ask, "Why are the news headlines so misleading?" It is not surprising that eating a diet with 29% of calories from fat does not improve health. It is not surprising that simply adding one serving of vegetables or fruit per day does not significantly improve health either! The attention that this research has received provides a good example of why we all need to be more informed about nutrition and health. It is important to "read between the lines".

Moving Forward

The way to improve your health and reduce your cancer risk is to eat a plant-based diet, maintain a healthy bodyweight, exercise regularly, and avoid tobacco (smoking and chewing). Ideally, the whole diet needs to be based around vegetables, fruit, whole grains, and legumes (beans). For true health benefit, no more than one-quarter of the plate should be covered by meat or other animal foods. This means that three-quarters of the plate should contain vegetables, fruit, whole grains, and legumes (beans). This style of eating will help you to eat at least 9 servings of vegetables and fruit each day. This type of diet will be naturally low in fat too.

The recent low-fat diet study confirms that very small changes in diet probably will make very small changes in health. Furthermore, studies in cancer survivors do tell us that eating a true low-fat diet and eating plenty of vegetables and fruit can lower cancer risk (17,18).

Thank you for your thoughtful question. We hope our answer is helpful to you. We hope that we have given you information that you can use to make informed choices regarding a healthy, cancer-fighting diet. We feel that helping people understand the science behind the many connections between what we eat and cancer is the best way to help them make wise nutrition choices!

If you are in cancer treatment, these diet changes may not be right for you.

References

1. The Women’s Health Initiative Study Group. Design of the Women's Health Initiative clinical trial and observational study. Control Clin Trials. 1998;19(1):61-109.
 
2. Chlebowski RT, Grosvenor M. The scope of nutrition intervention trials with cancer-related endpoints.
Cancer. 1994;74(9 Suppl):2734-38.

3. Prentice RL, Caan B, Chlebowski RT, Patterson R, Kuller LH, Ockene JK, Margolis KL, Limacher MC, et al. Low-fat dietary pattern and risk of invasive breast cancer: the Women's Health Initiative Randomized Controlled Dietary Modification Trial. JAMA. 2006;295(6):629-42.

4. Watzl B, Kulling SE, Moseneder J, Barth SW, Bub A. A 4-wk intervention with high intake of carotenoid-rich vegetables and fruit reduces plasma C-reactive protein in healthy, nonsmoking men. Am J Clin Nutr. 2005;82(5):1052-58.

5. Al-Delaimy WK, Ferrari P, Slimani N, Pala V, Johansson I, Nilsson S, Mattisson I, Wirfalt E, et. al. Plasma carotenoids as biomarkers of intake of fruits and vegetables: individual-level correlations in the European Prospective Investigation into Cancer and Nutrition (EPIC). Eur J Clin Nutr. 2005;59(12):1387-96.

6. Al-Delaimy WK, Slimani N, Ferrari P, Key T, Spencer E, Johansson I, Johansson G, Mattisson I et. al. Plasma carotenoids as biomarkers of intake of fruits and vegetables: ecological-level correlations in the European Prospective Investigation into Cancer and Nutrition (EPIC). Eur J Clin Nutr. 2005;59(12):1397-408. 

7. Broekmans WM, Klopping-Ketelaars IA, Schuurman CR, Verhagen H, van den Berg H, Kok FJ, van Poppel G. Fruits and vegetables increase plasma carotenoids and vitamins and decrease homocysteine in humans. J Nutr. 2000;130(6):1578-83.

8. Smith-Warner SA, Elmer PJ, Tharp TM, Fosdick L, Randall B, Gross M, Wood J, Potter JD. Increasing vegetable and fruit intake: randomized intervention and monitoring in an at-risk population. Cancer Epidemiol Biomarkers Prev. 2000;9(3):307-17.

9. Muller H, Bub A, Watzl B, Rechkemmer G. Plasma concentrations of carotenoids in healthy volunteers after intervention with carotenoid-rich foods. Eur J Nutr. 1999;38(1):35-44. 

10. Polsinelli ML, Rock CL, Henderson SA, Drewnowski A. Plasma carotenoids as biomarkers of fruit and vegetable servings in women. J Am Diet Assoc. 1998;98(2):194-96.

11. Paiva SA, Yeum KJ, Cao G, Prior RL, Russell RM. Postprandial plasma carotenoid responses following consumption of strawberries, red wine, vitamin C or spinach by elderly women. J Nutr. 1998;128(12):2391-94.

12. Rock CL. Carotenoids: biology and treatment. Pharmacol Ther. 1997;75(3):185-97. 

13. Rock CL, Flatt SW, Wright FA, Faerber S, Newman V, Kealey S, Pierce JP. Responsiveness of carotenoids to a high vegetable diet intervention designed to prevent breast cancer recurrence. Cancer Epidemiol Biomarkers Prev. 1997;6(8):617-23.

14. Yeum KJ, Booth SL, Sadowski JA, Liu C, Tang G, Krinsky NI, Russell RM. Human plasma carotenoid response to the ingestion of controlled diets high in fruits and vegetables. Am J Clin Nutr. 1996;64(4):594-602.

15. Le Marchand L, Hankin JH, Carter FS, Essling C, Luffey D, Franke AA, Wilkens LR, Cooney RV, Kolonel LN. A pilot study on the use of plasma carotenoids and ascorbic acid as markers of compliance to a high fruit and vegetable dietary intervention. Cancer Epidemiol Biomarkers Prev. 1994;3(3):245-51. 
 
16. Campbell DR, Gross MD, Martini MC, Grandits GA, Slavin JL, Potter JD. Plasma carotenoids as biomarkers of vegetable and fruit intake. Cancer Epidemiol Biomarkers Prev. 1994;3(6):493-500.

17. Abstract No. 10. Dietary fat reduction in postmenopausal women with primary breast cancer: Phase III Women's Intervention Nutrition Study (WINS). Available at: http://www.asco.org/ac/1,1003,_12-002643-00_18-0034-00_19-0031414,00.asp. Accessed May 27, 2005.

18. Rock CL, Flatt SW, Natarajan L, Thomson CA, Bardwell WA, Newman VA, Hollenbach KA, Jones L, Caan BJ, Pierce JP. Plasma carotenoids and recurrence-free survival in women with a history of breast cancer. J Clin Oncol. 2005;23(27):6631-38.

Publish Date: 02/2006